NI Circuit Design Community Blog
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PCB Transmission Line Calculator in Ultiboard
Hello Circuit Designers,
When creating PCB layouts, many routing aspects that may not be accounted for in the SPICE simulation should be accounted for. One of the most critical routing parameters affecting the quality of the signals on the PCB is impedance matching.
For low-frequency analog signals impedance matching is easy to achieve using simple buffer stages with high input impedance and low output impedance to maximize the power transfer at each stage of the design. However, when the frequency of the analog signal gets to the Mega-Hertz range, the PCB trace itself becomes a transmission line of a distributor nature. In simple words, the trace acts like a chain of series inductors and parallel capacitors. These elements are able to store energy and reflect it back unless the termination is a load resistance equal to the characteristic impedance of that transmission line (calculated by the below equation).
The same concept applies to all digital signals since they have an infinte span in the frequency domain.
The following two graphs display a 5V digital logic signal propagating through a 50-ohm transmission line. In the first graph the line is open-ended, the second case the line is terminated with a 42-ohm load (close to perfectly matched).
As you can see on the first graph, the reflection on the open-ended line caused a ripple of about 1V on the signal level which could propate back into the circuit and damage it.
To be able to create matched traces and avoid undesired reflections on the PCB, you need to answer these questions:
1. How thick is the PCB and what dielectric material is it made of?
2. What is the impedance of the load? (50 or 75 ohms in most of the cases)
Using these parameters, Ultiboard's transmission line calculator can help you determine the optimum width of the traces you are about to lay down on your board. Simply go to Tools>PCB transmission line calculator in Ultiboard's menu and enter these parameters as shown in the screenshot below.
(Note: choose the type of the transmission line based on where the trace the ground plane are. For example, pick Microstrip if the trace is on the top layer and the ground plane is on the first copper inner layer. In this case the board thickness will be the height (H) between these two layers)
Mahmoud W
National Instruments
